Chapter 4

Aqueous solutions

Types of reactions

mixture

homogeneous heterogeneous

one phaseTab water

two phasesoil/water

milk

solution

solvent Solute(s)more abundant component of

mixturewater in tab

water

Less abundant or other component(s) of mixturesalts in tab water

Water always solvent even in 98% H2SO4

solutes

ElectrolytesNon-

electrolytes

Solute undergoes Dissociation

strong weakcomplete

dissociationpartial

dissociationNaCl / H2OHCl / H2O

HAc / H2O

Sugar / H2O

)()()(2

aqaqOH

s ClNaNaCl )()()(2

aqaqOH

l AcHHAc

Hydration of Solid Solute

At edges, fewer oppositely charged ions around– H2O can come in; Ion-dipole forces; Remove ion

New ion at surface– Process continues until all ions in solution

Hydration of ions – Completely surrounded by solvent

4

Molecular Compounds In Water When molecules dissolve in water

– Solute particles are surrounded by water– Molecules are not dissociated

5

Electrical conductivityof electrolyte solutions

Ionic compounds

Strong acids and bases

Ex. NaBr, KNO3, HClO4, HCl, KOH

Weak acids and bases

Ex. Acetic acid (HC2H3O2), ammonia (NH3)

Ex. Sugar, alcohol

Learning Check

Write the equations that illustrate the dissociation of the following salts:

Na3PO4(aq) →

Al2(SO4)3(aq) →

CaCl2(aq) →

Ca(MnO4)2(aq) →7

3 Na+(aq) + PO43(aq)

Ca2+(aq) + 2 Cl(aq)

Solubility Maximum amount of a substance that can be

dissolved in a given amount of solvent at a given temperature.

Usually g/100 mL.

solvent g 100

solution saturated make toneeded gSolubility

solute

Saturated solution: Solution in which no more solute can be dissolved at a given temperatureUnsaturated solution: Solution containing less solute

than max. amount; Able to dissolve more solute

Solubilities of Some Common Substances

9

Substance FormulaSolubility

(g/100 g water)

Sodium chloride NaCl 35.7 at 0°C39.1 at 100°C

Sodium hydroxide

NaOH 42 at 0°C347 at 100°C

Calcium carbonate

CaCO3 0.0015 at 25°C

“Like dissolves Like” Ethanol (C2H5OH) dissolves in water:

polar ↔ polar

Glucose (C6H12O6) and sucrose (C12H22O11)

dissolve in water: polar ↔ polar

Oil doesn’t dissolve in water: nonpolar ↔ polar Oil dissolves in benzene: nonpolar ↔ nonpolar

Salts are polar.

soluble insolubleNaCl AgCl

Water unable to separate Ag+ from

Cl-

Interaction very strong

Relative Concentration

Dilute solution– Small solute to solvent ratio

Ex. Eyedrops

Concentrated solution– Large solute to solvent

ratio

Ex. Pickle brine

Dilute solution contains less solute per unit volume than more concentrated solution

12

Solute-to-solvent ratio

quantitatively

abbreviated M 1 M = 1 mol solute / 1 liter solution

Molarity

LV

nM

solution

solute

/

molmolg

g

M

mn

NaOH

NaOHNaOH 288.0

/0.40

5.11

ML

mol

L

mol

LV

nM

solution

NaOHNaOH 192.0192.0

50.1

288.0

/

Preparing Solution of Known Molarity

a b c d e

a) Weigh solid and transfer to volumetric flask

b) Add part of the water

c) Dissolve solute completely

d) Add water to reach etched line

e) Stopper flask and invert to mix thoroughly 14

Concentration of each type of ions in 0.50 M Co(NO3)2(aq)?

)(3)(2

)(23 2)( aqaqaq NOCoNOCo 1 mol 1 mol 2 mol

In 1.00 L 0.50 mol 0.50 mol 1.00 mol

Molarity 0.50 M 0.50 M 1.00 M

Concentration of each type of ions in 0.50 M Fe(ClO4)3(aq)?

)(4)(3

)(34 3)( aqaqaq ClOFeClOFe

Molarity 0.50 M 0.50 M 1.50 M

Moles of Cl- 1.75 L of 1.0×10-3 M ZnCl2(aq)?

molLL

moln

LMn

LVMnLV

nM

ZnCl

ZnCl

solutionZnClZnClsolution

ZnClZnCl

33

3

1075.175.1100.1

75.1100.1

//

2

2

22

2

2

)()(2

)(2 2 aqaqaq ClZnZnCl

1 2

1.75×10-3 mol ? moln

moln

Cl

Cl

3

3

105.3

1075.12

mol

gmolMnm

ClClCl45.351075.12 3

NaCl

NaClsolution

solution

NaClNaCl M

nV

LV

nM

/

molmolg

g

M

mn

NaCl

NaClNaCl

53

107.1/44.58

100.1

mLLLmol

mol

M

nV

NaCl

NaClsolution 12.0102.1

/14.0

107.1 45

Practice How many grams of HCl would be required to make

50.0 mL of a 2.7 M solution?

What would the concentration be if you used 27g of CaCl2 to make 500. mL of solution? What is the concentration of each ion?

Describe how to make 1.00 L of a 0.200 M K2CrO4 solution.

Describe how to make 250. mL of an 2.0 M copper (II) sulfate dihydrate solution.

Calculate the concentration of a solution made by dissolving 45.6 g of Fe2(SO4)3 to 475 mL. What is the concentration of each ion?

Describe how to make 1.00 L of a 0.200 M K2CrO4 solution.

molLMn

LVMnLV

nM

CrOK

solutionCrOKCrOKsolution

CrOKCrOK

200.000.1200.0

//

42

4242

42

42

gmolgmolMnm CrOKCrOKCrOK 8.58/20.294200.0424242

No solid K2CrO4 available in the

lab .But 2.00 M K2CrO4 solution is

available .

molLMn

LVMnLV

nM

CrOK

solutionCrOKCrOKsolution

CrOKCrOK

200.000.1200.0

//

42

4242

42

42

Dilution

2211

42424242

4242

VMVM

VMVM

VMn

nn

dilutionafterCrOKCrOKdilutionbeforeCrOKCrOK

dilutionafterCrOKdilutionbeforeCrOK

LM

LMV

LMVM

VMVM

100.000.2

00.1200.0

00.1200.000.2

1

1

2211

Prepare 150 mL of 0.100 M H2SO4 from 16.0 M solution.

mLM

mLMV

mLMVM

VMVM

938.00.16

150100.0

150100.00.16

1

1

2211

What volume of a 1.7 M solution is needed to make 250 mL of a 0.50 M solution?

18.5 mL of 2.3 M HCl is added to 250 mL of water. What is the concentration of the solution?

You have a 4.0 M stock solution. Describe how to make 1.0 L of a 0.75 M solution.

Types of Chemical Reactions

Metathesis Reduction-

OxidationDouble

Replacement

precipitation reaction

Acid-Base Reaction

precipitateFormation of a

weak electrolyte

a solid is formed from

solution

Electron transfer

Formation of a gas

AB + CD AD + CB

Precipitation reactions

Molecular equation

A reaction takes place if it has a net ionic equation

Ionic equation

)(4)(3)()(3)(2

)(2

4)( 2222 saqaqaqaqaqaq BaCrONOKNOBaCrOK

Net ionic equation: describes what really happens.

)(4)(2

4)(2

saqaq BaCrOCrOBa

Spectator ions: )(3)( 2, aqaq NOK

Solubility Rules1 All nitrates and acetates are soluble

2 Salts of alkali metals ions and NH4+ ions are soluble.

3 Chlorides, bromides and iodides (salts of Cl-, Br- and I-) are soluble except those of Ag+, Pb2+, and Hg2

2+.

4 Most sulfates are soluble, except those of Pb2+, Ba2+, Hg2+, and Ca2+.

5 Most hydroxides are slightly soluble (insoluble) except those of alkali metals (Ba(OH)2, Sr(OH)2 and Ca(OH)2 are marginally soluble).

6 Sulfides (S2-), carbonates (CO32-), chromates (CrO4

2-) and phosphates (PO4

3-), are insoluble except those of alkali metals and NH4

+.

Does the following mixing process involve a chemical reaction?

??)()(3 aqaq KClAgNO

3)()(3 KNOAgClKClAgNO aqaq

)(3)()()(3 aqsaqaq KNOAgClKClAgNO

)()()( saqaq AgClClAg

Precipitation reactions

NaOH(aq) + FeCl3(aq) ® ??

NaOH(aq) + FeCl3(aq) ® NaCl + Fe(OH)3

NaOH(aq) + FeCl3(aq) ® NaCl(aq) + Fe(OH)3(s)

Na+(aq)+OH-

(aq) + Fe3+ (aq) + Cl-(aq) ® Na+ (aq) +

Cl- (aq) + Fe(OH)3(s)

OH-(aq) + Fe3+

(aq) ® Fe(OH)3(s)

Precipitation reactions

BaCl2(aq) + KNO3(aq) ® ??

BaCl2(aq) + KNO3(aq) ® KCl + Ba(NO3)2

BaCl2(aq) + KNO3(aq) ® KCl(aq) + Ba(NO3)2(aq)

Ba2+(aq)+2 Cl-(aq) + K+

(aq) + NO3-(aq) ® K+ (aq) +

Cl- (aq) + Ba2+(aq)+ 2 NO3

-(aq)

No net ionic equation

No reaction

Practice iron (III) sulfate and potassium sulfide

Lead (II) nitrate and sulfuric acid.

solutions of NaOH and NiCl2 are mixed.

)(3)()()(3 aqsaqaq NaNOAgClNaClAgNO

molLMVMn AgNOAgNOAgNO 150.050.1100.0333

1 1

0.15 mol ? molnNaCl 150.0

gmol

gmol

Mnm NaClNaClNaCl

77.844.58150.0

1.25 L of 0.0500 M Pb(NO3)2 mixed with 2.0 L of 0.0250 M Na2SO4. Calculate the mass of

precipitate.??)( )(42)(23 aqaq SONaNOPb

)(3)(4)(42)(23 2)( aqsaqaq NaNOPbSOSONaNOPb

molLMVMn

molLMVMn

SONaSONaSONa

NOPbNOPbNOPb

0500.000.20250.0

0625.025.10500.0

424242

333 2)(2)(2)(

1 1

0.0625 mol ?

molnPbSO 0625.04

1 1

0.0500 mol ?

gMmm

molnPbSO

2.15

0500.04

Stoichiometry of Precipitation What mass of solid is formed when 100.00 mL

of 0.100 M Barium chloride is mixed with 100.00 mL of 0.100 M sodium hydroxide?

What volume of 0.204 M HCl is needed to precipitate the silver from 50.0 ml of 0.0500 M silver nitrate solution ?

25 mL 0.67 M of H2SO4 is added to 35 mL of 0.40 M CaCl2 . What mass CaSO4 Is formed?

Arrhenius Acid Substance that reacts with water to produce the

hydronium ion, H3O+

Acid + H2O Anion + H3O+

HA + H2O A– + H3O+

HC2H3O2(aq) + H2O H3O+(aq) + C2H3O2

−(aq)

HCl(g) + H2O Cl–(aq) + H3O+(aq)

Bronsted-Lowry Acid: H+ donor

Arrhenius Bases Substance that reacts with water to give OH–.

a. Metal Hydroxides

NaOH(s) Na+(aq) + OH–

(aq)

Mg(OH)2(s) Mg2+(aq) + 2OH–

(aq)

b. Basic Anhydrides

CaO(s) + H2O Ca(OH)2(aq)

Ca(OH)2(aq) Ca2+(aq) + 2OH–

(aq)

c. Molecular bases: NH3(aq)+H2O NH4

+(aq)+ OH-

(aq)

Bronsted Base: H+ acceptor

Acid-Base Reactions

??)()( aqaq NaOHHCl

)(2)()()( laqaqaq OHNaClNaOHHCl

Ionic equation

)(2)()()()()()( laqaqaqaqaqaq OHClNaOHNaClH

Weak electrolyte: H2O + H2O H3O+(aq)

+ OH-(aq)

Net ionic equation: )(2)()( laqaq OHOHH

Any strong acid + strong base

??)()(232 aqaq NaOHOHHC

)(2)(232)()(232 laqaqaq OHOHNaCNaOHOHHC

Formation of Weak electrolyte:

HAc + H2O H3O+(aq)+ Ac-

(aq)

Acid - Base Reactions are often called neutralization reaction Because the acid neutralizes the base.

Volume of 0.100 M HCl needed to neutralize 25.0 mL of 0.350 M NaOH ?

)(2)()()( laqaqaq OHNaClNaOHHCl

molLMVMn NaOHNaOHNaOH31075.80250.0350.0

1 1

? 8.75×10-3

molnHCl31075.8

L

Lmol

mol

M

nVVMn 2

3

1075.8100.0

1075.8

28.0 mL of 0.250 M HNO3 mixed with 53.0 mL of 0.320 M KOH;

1)Amount of water formed2)Concentrations of H+ and OH- at the end of rct

)(2)(3)()(3 laqaqaq OHKNOKOHHNO

mmolmLL

molVMn

mmolmLL

molVMn

KOHKOHKOH

HNOHNOHNO

0.170.53320.0

0.70.28250.0333

1 1

7.0 mmol ?

mmoln OH 0.72

1 1

17.0 mmol ? mmoln OH 172

HNO3 is Limiting reactant: reacts completely

1)No HNO3 left

2)HNO3 → H+ + NO3-

3)No H+ at the end of reaction

MmLV

nM

solution

HH

05328

0

How much remains from KOH?

1)KOH → K+ + OH-

10 → 10 mmol

mmolnnn

nnn

reactedHNOinitiallyKOHremainingKOH

reactedKOHinitiallyKOHremainingKOH

1070.173

MmL

mmol

mL

mmol

V

nM

solution

OHOH

123.081

10

5328

10

Volumetric analysis: TitrationControlled addition of 1 reactant to another until rxn is complete.

Acid-Base Titration: Very common type of titrationEx. Analysis of citric acid in orange juice by neutralization

with NaOH

An indicator is needed: organic substance that changes color according to solution acidity

Where the indicator changes color is the endpoint.

Endpoint must be very close to the equivalence point.Acid (Base) added equivalent to base (acid) present

PhenolphthaleinAcidic Basic

baseacid ntoequivalentn

Standardization of NaOH solution• Know the exact concentration!• Its weight is inaccurate .

• NaOH is hygroscopic and it absorbs CO2.

• Cannot be used to prepare solutions with exactly known M.

• Not a primary standard.• KHP is a primary standard: high purity, no weighing

problems, Potassium hydrogen phthalate: KHC8H4O4.

Monoprotic!41.2 mL of NaOH solution is needed to react exactly with 1.300 g of KHP

(M=204.22 g/mol). MNaOH=?

ML

molg

g

VMwt

mM

Mwt

mVM

nn

NaOHKHP

KHPNaOH

KHP

KHPNaOHNaOH

KHPNaOH

1546.004120.022.204

300.1

%82.56%1003518.0

1999.0%100%

1999.012.12201059.01546.0

g

g

m

macid

gmol

gL

L

molm

MwtVMm

Mwt

mVM

nn

sample

acid

acid

acidNaOHNaOHacid

acid

acidNaOHNaOH

acidNaOH

practice 75 mL of 0.25M HCl is mixed with 225 mL of

0.055 M Ba(OH)2 . What is the

concentration of the excess H+ or OH- ?

A 50.00 mL sample of aqueous Ca(OH)2 requires 34.66 mL of 0.0980 M Nitric acid for neutralization. What is [Ca(OH)2 ]?

32 2

1)( HNOOHCa nn

wheneEquivalenc